Carburator Clarification... Does the "U" really make all the difference?

Hello All,

My 51 Buickmanual indicates the AAUVB-267 is for series 40-50 and the AAVB-267 is for series 70. It states they are not interchangable due to engine performance even though they appear to be identical. Is this true as I have both carbs but the AAVB-267 is in better shape. I have a 51 Special 40 series. What issues will it cause? Can you use either or?

Manual dosen't seem to identify the specific differences.

I would guess the jets/ports between the two are a bit different? Large port/jets dump more fuel. The answer might lie in the difference in the two engines. Bore and stroke, etc. If there is one.

Thanks. I'll have to see if it makes a difference as I believe the 70 series had teh larger straight 8?

The larger straight 8 I would imagine had more fuel delivered via the carb than the small straight 8. This then would be reason the two carbs are not interchangeable. The small of the two would get to much fuel from carb that is jetted for the larger straight 8. Conjecture on my part but highly probable.

The 40/50 carburetor is much smaller internally (the "U" means 1/8 inch undersize).

What will happen if you interchange?

Put the 40/50 carb on the 70 and it will run too rich up to the point where it just plain runs out of air (at maybe half throttle).

Put the 70 carb on the 40/50 and it will run too lean because of low internal air velocity.

Jon.